1. Field of the Invention
The present invention is related to a tuning indicator system for an FM radio receiver.
2. Description of the Prior Art
In FM radio receiver, unless this receiver is tuned exactly to a given station signal frequency, serious distortion and poor channel separation would result. Therefore, most of known FM radio receivers incorporate some form of visual tuning indicator system as an aid for correct tuning, to avoid the above-said inconveniences.
Conventional tuning indicator systems employed in general may be roughly classified into the following three types: signal meter type, center-tuning meter (S-meter) type, and their combined type.
The signal meter type is such that an indicator meter is driven to indicate the level of carrier signal taken at the IF (Intermediate Frequency) amplifier stage of the receiver. When the receiver is tuned correctly to a station signal, the carrier signal attains a peak level, and then a peak deflection is observed on the indicator meter. With this type of tuning indicator system, however, there often arises the accompaniment of the inconvenience that, in case a station signal is received at a high field strength, the peak point of the carrier signal level and accordingly the peak of the meter deflection could become indistinct due to the signal-limiting action by a limiter in the IF stage, which would obstruct sharp tuning.
The center-tuning meter type system, on the other hand, is arranged so that an indicator meter is driven in accordance with the dc-voltage level of the output of the FM demodulator in the receiver. However, there is the inconvenience that the meter will indicate null point, i.e. the center point in both accurate tuning-in and complete detuning conditions of the receiver, giving rise to confusion.
The combination of the above-mentioned two types of system may be considered to obviate such disadvantages as those that have been stated above from using them separately. However, it may, in turn, give rise to a new problem, which is troublesome tuning operation due to the necessity to simultaneously observe two different types of meters for tuning indication; one meter indicating the carrier signal level of the IF stage, and the other meter indicating the dc-voltage level of the FM-demodulator output.